My knowledge of Blue Archive lore/world-building is very much incomplete, but this is a canon side-story about how Sensei can't defend himself if just a few "basic enemies" decide to assault him. (I'm assuming that the Halos give them superhuman strength and endurance to some extent. Very least they protect from bullets, prologue has a "show, don't tell" -moment where a character takes bullet spray to the face and is more annoyed than anything.